1. Field of the Invention
This invention relates to device testing and, more particularly, to measuring, for example, the extinction ratio and the deterministic jitter of optical devices.
2. Description of the Related Art
Serial communications links are becoming increasingly popular due to a reduction in the bandwidth and physical limitations that may be imposed by some clock and data bus transmission systems. However, in a high-speed serial communications link, data integrity may be reduced by a problem known as jitter. Jitter may be generally described as a mis-positioning of the significant edges in a sequence of data bits from their ideal positions. If the jitter is significant, data loss may result. Jitter may be generally characterized by two types: deterministic jitter and random jitter. Deterministic jitter is due to non-Gaussian events and is bounded in amplitude. Deterministic jitter has different causes, such as: data dependence, duty cycle distortion, sinusoidal and uncorrelated (to the data). Random jitter is jitter that is Gaussian and it is unbounded. Accordingly, total jitter is the sum of the peak-to-peak values of the deterministic and random jitter.
One example of a serial communication link is an optical fiber link. Optical fiber is fast becoming the main choice for communications due to its signal carrying capabilities. To communicate via an optical fiber link a device commonly referred to as an optical transceiver may be used. An optical transceiver is a device that is used to transmit and receive optical signals through an optical medium such as a fiber optic cable. In most cases, an optical signal must be converted to an electrical signal. To accomplish this, an interface adapter may be necessary. Fiber optic cable and optical links are used in many applications. One such application is a fibre channel application. In a typical fibre channel application, an interface adapter may be used to convert an electrical signal to an optical signal and to transmit that optical signal. The interface adapter may also be used to convert a received optical signal to an electrical signal. Devices that perform these operations may conform to the Media Interface Adapter (MIA) specification and may be referred to as fibre channel MIA devices.
FIG. 1 illustrates a block diagram of one example of a fibre channel MIA device such as, for example, an MDB-9-8-X Fibre Channel MIA manufactured by Methode Electronics, Inc. In FIG. 1, fibre channel MIA device 100 contains both a transmit circuit 110 and a receive circuit 120. This particular example includes an electrical interface 130 and an optical interface 140. Electrical interface 130 may allow for an electrical connector 160 such as, for example, a DB9 connector. Optical interface 140 may allow for direct connection to optical fiber 150, or in some cases, an optical fiber connector (not shown). Transmit circuit 110 may contain a light emitting source such as a laser. It is noted that the fibre channel device of FIG. 1 is only one example of a fibre channel device and that there may be many other devices which may be used.
As described above, the effects of excessive jitter may be a problem in some serial communications links. To ensure that a device, such as fibre channel MIA device 100, does not introduce excessive jitter, it may be tested to determine its jitter. Fiber channel characteristics such as jitter and jitter compliance are outlined in documents such as the Fiber Channel Physical Interface Specification (FC-PI) available from the T11 committee within the National Committee for Information Technology Standards.
To test the data dependent jitter (DDJ) portion of the deterministic jitter of an interface device, some typical test systems may use an eye diagram method and an oscilloscope in a persistent display capture mode. This method may have some inaccuracies due to such factors as scope settings, source laser power output and incorrect extinction ratio settings. Additionally, the eye diagram may contain random jitter components. Therefore it is desirable to have a fast accurate way to measure DDJ using a digitizing oscilloscope.
Various embodiments of a system and method of measuring extinction ratio and deterministic jitter of an optical transceiver are disclosed. In one embodiment, the measurement system includes a computing node and an oscilloscope coupled to the computing node. The oscilloscope is also coupled to the optical transceiver. The oscilloscope is configured to capture a waveform of a predetermined data pattern transmitted by the optical transceiver. The oscilloscope is configured to capture the waveform in a non-persistent mode using waveform averaging. The oscilloscope is also configured to perform measurements on the waveform. The computing node is configured to communicate with the oscilloscope and to program the oscilloscope to perform the measurements on the waveform. The computing node is also configured to calculate an extinction ratio and to compare the extinction ratio to an acceptable standard. The computing node is also configured to calculate a deterministic jitter value of the optical transceiver in response to the extinction ratio being within the acceptable standard.
In other embodiments, the system may measure the extinction ratio as a ratio of an amplitude of a binary one to an amplitude of a binary zero. The amplitude of said binary one is measured on a non-oscillatory portion of a peak of said binary one and said amplitude of a binary zero is measured on a non-oscillatory portion of a peak of said binary zero. Further, the oscilloscope is configured to measure a position of each of a plurality of data pulse edges corresponding to the pattern of data with respect to a zero reference position. The oscilloscope is also configured to measure the position of each of the plurality of data pulse edges at a 50% crossing point. The 50% crossing point is an average power point of the waveform as measured using a filter.
In an additional embodiment, the system is configured to calculate a deterministic jitter value by first calculating a plurality of positive and negative jitter components corresponding to the plurality of data pulse edges. Next, the system finds a first maximum value of the positive jitter components and a second maximum value of the negative jitter components. Then the system calculates a first absolute value of the first maximum value of the positive jitter components and a second absolute value of the second maximum value of the negative jitter components. The system may then add the first absolute value to the second absolute value.
In addition, a method of operating a measurement system for characterizing an optical transceiver including a computing node and an oscilloscope is contemplated. In one embodiment, a waveform of a predetermined data pattern transmitted by said optical transceiver in a non-persistent mode using waveform averaging is captured. Further, an extinction ratio is calculated and compared to an acceptable standard. A deterministic jitter value of the optical transceiver is then calculated in response to the extinction ratio being within the acceptable standard.